Coherent optical detector and coherent communication system and method

ABSTRACT

An optical device is provided with first and second inputs. A first coupler coupled is coupled to the first input and produces at least a first and second output. A second coupler is coupled to the second input and produces at least a first and second output. A third coupler is coupled to the first output of the first coupler and to the first output of the second coupler. A fourth coupler is coupled to the second output of the first coupler and to the second output of the second coupler. First and second crossing waveguides are provided with an angle selected to minimize crosstalk and losses between the first and second cross waveguides. The first crossing waveguide connects one of the first or second outputs from the first coupler with an input of the fourth coupler. The second crossing waveguide connects one of the first or second outputs from the second coupler with an input of the third coupler. A first phase shifter is coupled to the first and second waveguides. The first and second waveguides connect one of the outputs of the first or second coupler and one of the inputs of the third or fourth couplers. The first, second, third and fourth couplers, the two crossing waveguides and the phase shifter are each formed as part of a single planar chip made of an electro-optical material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/669,130. Thisapplication claims the benefit of the Provisional Patent ApplicationSer. No. 60/462,348 filed Apr. 11, 2003, and is also acontinuation-in-part of U.S. patent application Ser. No. 09/962,243filed Sep. 26, 2001 and No. 10/613,772 filed Jul. 2, 2003, all of whichapplications are fully incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to integrated electro-optical receivingdevice and their methods and use, and more particularly toelectro-optical receiving devices formed on a single chip, and theirmethods of use, as well as optical communications systems and methodsthat utilize their coherent detection.

BACKGROUND OF THE INVENTION

Lasers are widely used today in fiber and free space segments for highdata rate communication links, remote sensing applications (LIDAR) andmore. In these applications the modulated light signal is modulatedusing electro-optical modulators and demodulated using, usually,electro-optical receiving devices.

In optical communications the modulation scheme commonly used is On-OffKeying (OOK), as illustrated in FIG. 1(a), where only the power of thelight is modulated. Alternative modulation schemes include Phase ShiftKeying (PSK), where the data is encoded in the phase of the signal. InRF communications more advanced modulation schemes are used, such asQuadrature Phase Shift Keying) (QPSK) and Quadrature AmplitudeModulation (QAM) see FIG. 1(b).

By using such communication schemes, for example, in opticalcommunication systems, the capacity and link performance can be greatlyenhanced in comparison with the direct detection schemes. In LIDAR,which is the extension of radar to the optical domain, the requiredshaping of the pulses can be achieved, such as chirped pulses, Barkercoding, etc.

Such modulating formats as PSK (for example, BPSK and QPSK) were usedmostly in the coherent communication systems (see, for example, L. G.Kazovsky, “Phase- and Polarization-Diversity Coherent OpticalTechniques”, J. of Lightwave Technology, Vol 7, N2, pp. 279-292, 1989).The majority of the work in this field was made by implementingnon-integrated solutions, i.e. various optical components such asamplitude and phase modulators connected by optical fibers. Suchcommunication schemes were abandoned in the late 1980's and are stillnot implemented due to their complexity and high cost.

For these applications and others, the light should be modulated both inamplitude and phase, essentially with a complex modulation signal. Thecost is increased complexity of receivers in coherent detection schemes.The main problem is related to the development of compact, reliable, andlow-cost receivers for such advanced modulating schemes.

At the receiver the received optical signal is mixed with the localoscillator signal by an optical interface that is usually based on oneor more optical hybrids, such as directional hybrids, polarizationsplitters, and 90-degrees balanced hybrids. At the output from theoptical interface, the optical field is converted into electric currentsby one or more PIN photodiodes.

If the local oscillator and the received optical carrier have the samefrequency, the electric currents provided by the photodiodes arebaseband signals and the receiver is of the homodyne type. Respectively,if the local oscillator and the received optical carrier have differentfrequencies, the electric currents are shifted to the intermediatefrequency (IF).

The present invention relates generally to the integrated phasediversity optical receiver designated to detect the optical signal, tomix it with another optical signal, to transform the signal intoelectrical domain for further processing. In-phase and in-quadraturedetection can be applied to coherent optical receiver as a technique tomeasure simultaneously the phase and amplitude of the optical field, seefor example, N. G. Walker, J. E. Carroll, “Simultaneous phase andamplitude measurements on optical signals using a multiport junction”,Electronics Letters, Vol. 20, N23, 981-983, 1984 and T. G. Hodkinson, etal., “Demodulation of optical DPSK using in-phase and quadraturedetection”, Electronics Letters, Vol. 21, N19, 867-868, 1985.

Optical devices currently available are based on non-integrated and/orsemi-integrated solutions, i.e. optical fibers or optical fiber-basedcomponents are used for connecting of various electro-optical componentsand/or splitting/combining the optical signals. There are no completelyplanar integrated solutions for the device that are capable to providean arbitrary format demodulation (phase and/or amplitude modulation).

Other work in monolithic integration of some optical receivers havedifferent implementations and/or are still far from being implemented inpractical optical systems (see, for example, J. Saulnier et al. “Opticalpolarization-diversity receiver integrated on Titanium-diffused LithiumNiobate”, IEEE Photonics Technology Letters, v.3. #10, 1991; F. Ghirardiet al. “InP-based 10 Ghz Bandwidth Polarization diversity heterodinephotodetector with electrooptical adjustability,” IEEE PhotonicsTechnology Letters, v.6. #7, 1994; D. Hoffman et al. “Integrated opticseight-port 90-degrees hybrid on LiNbO₃” Journal of Lightwave technology,v.7. #5, 1989, pp. 794-798).

Accordingly, there is a need for integrated monolithic devices thatprovide demodulating of quadrature phase shift keying modulated signal(BPSK and/or QPSK) or quadrature amplitude modulated signal (QAM) by useof a single, monolithically integrated device. There is a further needfor improved devices that is re-applicable for BPSK and/or QPSKcommunication systems, for LADAR as well as other remote sensingapplications.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anelectro-optically adjustable optical device, its methods of use thatmeasure simultaneously the phase and amplitude of the optical field.

Another object of the present invention is to provide an integratedsingle monolithic adjustable device to demodulate optical signals.

Yet another object of the present invention is to provide an integratedsingle monolithic adjustable device to demodulate optical signals forquadrature phase shift keying (BPSK and/or QPSK) optical datacommunication applications.

A further object of the present invention is to provide an integratedsingle monolithic adjustable device to demodulate optical signals forquadrature amplitude modulation (QAM).

Still another object of the present invention is to provide anintegrated single monolithic adjustable device to demodulate opticalsignals for LADAR as well as other remote sensing applications.

Another object of the present invention is to provide a velocity findingsystem which utilizes a coherent communication incoming signal frequencydeviation that corresponds to the Doppler shift to calculate theVelocity of a transmitting device with respect to the receiving device.

Yet another object of the present invention is to providesmaneuverability measurement system that measures the incoming signalpolarization deviation with respect to a receiving device.

A further object of the present invention is to provide a range findingsystem that compares GPS time stamps embedded in an incoming signal datastream with receiver time ticks locally that are generated by the GPSsignal.

These and other objects of the present invention are achieved in anoptical device with first and second inputs. A first coupler coupled iscoupled to the first input and produces at least a first and secondoutput. A second coupler is coupled to the second input and produces atleast a first and second output. A third coupler is coupled to the firstoutput of the first coupler and to the first output of the secondcoupler. A fourth coupler is coupled to the second output of the firstcoupler and to the second output of the second coupler. First and secondcrossing waveguides are provided with an angle selected to minimizecrosstalk and losses between the first and second cross waveguides. Thefirst crossing waveguide connects one of the first or second outputsfrom the first coupler with an input of the fourth coupler. The secondcrossing waveguide connects one of the first or second outputs from thesecond coupler with an input of the third coupler. A first phase shifteris coupled to the first and second waveguides. The first and secondwaveguides connect one of the outputs of the first or second coupler andone of the inputs of the third or fourth couplers. The first, second,third and fourth couplers, the two crossing waveguides and the phaseshifter are each formed as part of a single planar chip made of anelectro-optical material.

In another embodiment of the present invention, a method of transmissionincludes providing an optical device with first, second, third andfourth couplers. The two crossing waveguides and the phase shifter areeach formed as part of a single planar chip made of an electro-opticalmaterial. A voltage is applied to the first, second, third and fourthcouplers to maintain a desired power splitting ratio.

In another embodiment of the present invention, an optical communicationsystem includes a transmitter and a receiver. The receiver includes, (i)a first coupler coupled to the first input and produces at least a firstand second output, (ii) a second coupler coupled to the second input andproduces at least a first and second output, (III) a third couplercoupled to the first output of the first coupler and to the first outputof the second coupler, (iv) a fourth coupler coupled to the secondoutput of the first coupler and to the second output of the secondcoupler, (v) first and second crossing waveguides with an angle selectedto minimize crosstalk and losses between the first and second crosswaveguides, the first crossing waveguide connecting one of the first orsecond outputs from the first coupler with an input of the fourthcoupler, the second crossing waveguide connecting one of the first orsecond outputs from the second coupler with an input of the thirdcoupler. A first phase shifter is coupled to the first and secondwaveguides. The first and second waveguides are connected to one of theoutputs of the first or second coupler and one of the inputs of thethird or fourth coupler. The first, second, third and fourth couplers,the two crossing waveguides and the phase shifter are each formed aspart of a single planar chip made of an electro-optical material.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(a) illustrates the optical communication modulation format OOK.

FIG. 1(b) illustrates the optical communication modulation format (b)QAM.

FIG. 2(a) is a schematic diagram of one embodiment of a 90-degreesoptical hybrid of the present invention.

FIG. 2(b) is a schematic diagram illustrating a quadrature receiver ofthe present invention.

FIGS. 3 a and b illustrate that the tunable 3 dB couplers from FIG. 2(b)can be based on an alternating Δβelectrode geometry.

FIG. 4 illustrates one embodiment of a coherent receiver of the presentinvention.

FIG. 5 illustrates one embodiment for a digital calibration scheme forthe FIG. 4 coherent receiver.

FIG. 6 illustrates one embodiment for an analog self-homodynecalibration scheme for the FIG. 4 coherent receiver.

FIG. 7 illustrates angular misalignment, or non-bore sighting, betweenthe Tx and Rx apertures that leads to a continuous time delay of thesignals across the optical beam at the detector.

FIG. 8(a) is a simulated eye diagram of the received RZ-QPSK signal withCW LO and delay of 1.3258 of the symbol period (106.1 ps). Q²=16.3 dB.

FIG 8(b) is a simulated eye diagram of the received RZ-QPSK signal withpulsed LO and delay of 1.3258 of the symbol period (106.1 ps).

FIG. 9 is a schematic diagram of an optical communication system of thepresent invention.

FIG. 10(a) illustrates one embodiment of a transmitter of FIG. 9 thatincludes Mach-Zehnder modulators.

FIG. 10(b) illustrates an embodiment of a transmitter of FIG. 9, wherethe transmitters are quadrature modulators operating with the light intwo (orthogonal) polarization states.

FIG. 11 illustrates a fully coherent digital free-space-opticalcommunication link embodiment of the present invention.

FIG. 12 illustrates an application of an embodiment of the presentinvention for maneuverability control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the present invention, an optical device, such asoptical device 200 illustrated in FIG. 2(a), hereafter the “OpticalDevice”, is provided that includes a first device input, and a seconddevice input, first, second, third and fourth couplers (mixers) andfirst, second, third and fourth device outputs. Each coupler includestwo adjacent waveguides providing the mixing of the optical signalspropagating in these adjacent waveguides. The first and second deviceinputs both are connected, respectively, to the first coupler and thesecond coupler.

One output of the first coupler is connected to one input of the thirdcoupler while another output of the first coupler is connected to theone input of the fourth coupler. An output of the second coupler isconnected to another input of the fourth coupler while another output ofthe first coupler is connected to another input of the third coupler.The Optical Device further includes two crossing waveguides, which crosseach other. The first crossing waveguide connects the one output of thefirst coupler and another input of the fourth coupler. The secondcrossing waveguide connects the one output of the first coupler andanother input of the third coupler. The Optical Device also includes atleast one phase shifter positioned between two locations. The firstlocation is one of the outputs of the first or second coupler. The otherlocation is one of the inputs of the third or fourth couplers, whichcorresponds (connected by a crossing waveguide) to the first location.The first and second outputs of the third coupler produce the first andsecond device outputs, respectively. The first and second outputs of thefourth coupler produce the third and fourth device outputs,respectively.

The first, second, third and fourth couplers, two crossing waveguidesand the phase shifter can each be formed as part of a single planar chipmade of an electro-optical material. A variety of different processesare utilized in making the single planer chip, as disclosed in R. C.Alferness in “Ti Diffused LiNbO₃ waveguide devices”, “Guided WaveOpto-electronics”, Ed. T. Tamir, Springer-Verlag, 1988; and Wei-LinChen, et al. “Lithium Niobate Ridge Waveguides by Nickel Diffusion andProton-Exchange and Wet Etching”, IEEE Photonics Technology Letters Vol.7 No. 11, 1995, all incorporated herein by reference.

In various embodiments, the chip is a monolithic piece of a wafer thatcan be made of semiconductor or ferroelectric materials including butnot limited to LiNbO₃, and the like. In various embodiments, differenteffects relative to the output of the chip of the present invention arepossible, including but not limited to, (i) thermo-optical, (ii)electro-optical, (III) electro-absorption, and the like can be utilizedwith the Optical Device. The electro-optical material, which can beLiNbO₃, can be cut at X, Y, or Z planes. The Optical Device of thepresent invention can utilize a variety of different processes in itscreation, including but not limited to, metal in-diffusion and/or(annealed) protonic-exchange technology, wet etching, reactive ion(beam) etching, plasma etching, and the like.

Integration of components in a single chip, such as LiNbO₃ and the like,can, among other things, reduce cost, improve performance, and providebetter stability and control. The Optical Device of the presentinvention, when integrated on a single chip and/or in single package,can be used for various applications, including those that requiresimultaneous measurement of phase and amplitude of the optical field.

In one embodiment of the present invention, the optical input of theOptical Device has as input signal that is modulated in phase/amplitude,such as by way of illustration, quadrature phase shift keying (QPSK) orquadrature amplitude modulation (QAM), for communications, or controlledchirp or Barker coding for LADAR applications.

FIG. 2(a) illustrates an embodiment of Optical Device 100 that is a90-degrees optical hybrid. The chip can be cut at the X plane. In thisembodiment, the single chip, made of electro-optical material, thebalance receivers and/or TIAs are all formed a part of a singleintegrated package.

In this embodiment, the two incoming light signals impinge the OpticalDevice 100 through the inputs 104 and 105, each subsequently dividedinto two optical signals. The division can be achieved by a variety ofdifferent ways, including but not limited to use of 3 dB couplers 106and 107, respectively. The light from coupler 106 further directed tothe couplers 108 and 109 and light from coupler 107 further directed tothe couplers 108 and 109, in such a way that the crossing waveguidestructure 111 is produced. The two, preferably 3 dB, couplers 108 and109 further mix the corresponded input signals and produce four opticaloutputs 112, 113, 114 and 115. At least one phase shifter 110 is placedon waveguide structure 111 in-between the first set of the couplers 106and 107 and the second set of the couplers 108 and 109. The beam whichpasses through the phase shifter 110 gains the additional phase shift.Output optical fields 112, 113, 114, 115 are converted into electriccurrents by four PIN photodiodes 102. It will be appreciated that deviceother than the four PIN photodiodes 102 can be utilized. Each pair ofbalanced receivers is connected to a corresponding trans-impedanceamplifier 103.

A block diagram of a “quadrature receiver” embodiment of an OpticalDevice 200 of the present invention, is shown in FIG. 2(b). Two incomingoptical signals 201 and 202, called, respectively, the signal S and thelocal oscillator L, impinge two inputs 203 and 204 of the opticalhybrid. Passive couplers or splitters 205, 206 divide the light comingfrom input S and L into four, preferably equal beams 207, 208, 209 and210.

The beam 207 passes through phase shifter 221 and gains the additionalphase shift. It will be appreciated that additional phase shifters 221and gains the additional phase shift. It will be appreciated thatadditional phase shifters 221 can be included. Additional bias can beapplied to phase shifter 221 in order to obtain the desirable phaseshift.

Beams 211 and 212 are mixed together by directional coupler 215. Beams213 and 214 are mixed together, respectively, at the directional coupler216. Couplers 215 and 216 intrinsically introduce the 90-degrees phaseshift between two outcoming signals. Bias voltages can be applied toeach coupler 205, 206, 215 and 216 to set the 3 dB splitting operatingpoint.

The resulting four output signals A, B, C, D, that come from outputs217, 218, 219 and 220, can all have an adjustable relative phasedifference with respect to each other. The first two outputs can providethe cosine of the relative phase between S and L after balanceddetectors. The last two outputs can provide the sine of the relativephase.

If couplers 205, 206, 215 and 216 all are 3 DB couplers, and the singlephase shift 221 provides 90-degrees phase shift, then all four outputs211, 212, 213 and 214 have 90-degrees relative phase difference of theform:

-   -   {A=S+L, B=S−L, C=S+jL D=S−jL}.

The outputs of the S and L after balanced detector are sampled andprocessed.

Couplers 206, 207, 215 and 216 can be based on an alternating Δβelectrode geometry as illustrated in A. Yariv and P. Yeh, Optical wavesin crystals, Chapter 11, Wiley-Interscience, incorporated herein byreference, in order to ensure the occurrence of power crossover withinfabrication tolerance of couplers 205, 206, 215 or 216. In thisconfiguration, each coupler 205, 206, 217 and 216 has electrodes 310(FIG. 3 a, b) that are split into two parts in a manner so that thevoltage has a reversed polarity at each section.

When couplers 205, 206, 215 and 216 are 3 dB couplers, the 3 dB-workingpoint of each coupler 205 through 216 is optimized from considerationsof optimal sensitivity to applied voltage and reduced sensitivity tovariations in diffusion conditions, as more fully illustrated in FIG. 3a, b. This makes couplers 205 through 216 less sensitive to diffusionvariations, resulting in an increase in manufacturing yield.

Referring now to FIG. 4, one embodiment of the present invention is acoherent receiver 400 that includes a 90-degrees optical hybrid 410 and2 pairs of balanced detectors 430 and 440. Optical hybrid 410 has twooptical inputs 401 and 402 and four optical outputs 411, 412, 413, 414.When two optical signals S and L are provided to inputs 401 and 402, thefollowing optical signals are at outputs 411, 412, 413 and 414::S+L,S−L, S+jL, S−jL respectively. Optical hybrid 410 can have variousconfigurations. In one embodiment, optical hybrid 410 has fourdirectional couplers 403, 404, 407, 408 and two phase shifters 405, 406.Regulating voltages 409, 415, 418, and 419 are applied to each of thedirectional couplers to assure equal power splitting between its twooutputs by controlling the “coupler phase” β_(i). Regulating voltages416 and 417 are applied to phase modulators 405 and 406 in order tocontrol the phase shifting angle by controlling the phase shiftparameter γ_(i). Each balanced detector 430 and 440 has a pair ofbalanced (equal characteristics) photodiodes 431, 432 and 441, 442,followed by a Trans-Impedance Amplifier (TIA) 435 and 445 respectively.The output signals from coherent receiver 400 come out via theelectrical outputs 428 and 429.

In one embodiment, referred to as the digital implementation, theoutputs of balanced detectors 430 and 440 are sampled and processed. Inthe analog self-homodyne implementation, the inputs are the receivedsignal S and a delayed replica of it delayed by one symbol S_(d).Outputs 428 and 429 provide the real and imaginary part ofSS_(d)*e^(−jπ/4). Each output 428 and 429 only need to be compared tozero in order to recover the transmitted bit.

Calibration maintains certain performance parameters of the coherentdetector internal subassemblies as follows:

For directional coupler 403, equal splitting of light energy intooutputs 421 and 422 can be achieved by controlling the ‘coupler phase’β₁ via control line 409.

For directional coupler 404, equal splitting of light energy intooutputs 423 and 424 can be achieved by controlling the ‘coupler phase’β₂ via control line 415.

For directional coupler 407, equal splitting of light energy intooutputs 411 and 412 can be achieved by controlling the ‘coupler phase’β₃ via control line 418.

For directional coupler 408, equal splitting of light energy intooutputs 413 and 414 can be achieved by controlling the ‘coupler phase’β₄ via control line 419.

For phase shifters 405 and 406, 90-degrees relative phase differencebetween inputs 423 and 425 (e.g. S and L) and 422 and 426 (e.g. S andjL), and for the analog self-homodyne implementation to ensure that theconstellation absolute alignment is correct (e.g. optical outputs S ande^(jπ/4Sd), and S and je^(iπ/4Sd)), can be achieved by controlling thephase shift parameters γ₁ and γ₂ via control line 416 and 417.

Coherent receiver 400 can be calibrated digitally, as FIG. 5, or in ananalog self homodyne manner, as illustrated in FIG. 6 with thecalibration block marked as 446. Referring to FIG. 5, coherent receiver400 is digitally calibrated. Coherent detector outputs 428 and 429 canbe connected to two fast (symbol rate) A/D converters for furtherdigital signal processing. Digital samples 448 and 450 are processed byprocessor 453.

In the analog self-homodyne implementation embodiment, illustrated inFIG. 6, coherent detector analog outputs 428 and 429 can be tapped andsampled by two A/D converters 455 and 456. The A/D outputs are directedto processor 453. These A/Ds can have bandwidths lower than the symbolrate, since they are used while keeping the symbols constant during theA/Ds integration time (training).

Processor 453 collects samples of inputs 448 and 450, and estimatestheir statistical properties, and performs the control algorithms asdescribed below.

The algorithm results are applied to a set of controllers. The processorcontrols the coupler phases β_(i) via the coupler phase controller 452.The phase shifts γ_(i) are controlled by 454.

The calibration process is divided into two stages: (i) initialization,to be performed at the Optical Device start-up and resets, and (ii)online tracking, carried out continuously during operation (on trainingsequences or on actual payload). When optical polarization compensationis used, the initialization stage assumes that it is set correctly. Theinput signal to the Optical device of the present invention includesdata points that have all constellation points with equal probability.In one embodiment, the data points have constellation points with amaximum length sequence. In the analog implementation embodiment, thesymbols are kept constant during the A/Ds integration time.

In one embodiment, the following algorithm is used to initialize thecoupler phase (and therefore ensure equal splitting of light energy atthe coupler output) of 407 408 and 403 and 404 (in that order). It willbe appreciated that the present invention is not limited to thesealgorithms, and that a variety of other algorithms can be used.

Initializing 407 ‘coupler phase’ to π/4+k₁π/2, where k₁ is an integer(representing a π/2 ambiguity), can implement the following:

1. Adjust control line 409 via controller 452, to maximize the varianceof output 428.

2. Adjust control line 409 via controller 452, to maximize the varianceof output 428.

3. Adjust control line 415 via controller 452, to maximize the varianceof output 428.

Initializing 408 ‘coupler phase’ to π/4+k₂π/2 can utilize the following:

1. Adjust control line 409 via controller 452, to maximize the varianceof output 429.

2. Adjust control line 415 via controller 452, to maximize the varianceof output 429.

3. Adjust control line 419 via controller 452, to maximize the varianceof output 429.

The initialization of both 403 and 404 ‘coupler phase’ to π/4+k₃π/2,π/4+k₄π/2 can utilize the following:

1. Adjust control lines 409 and 415 at the same time via controller 452,to maximize the sum of the two variances at the outputs 428 while,

2. Maintaining the variance of output 428 equal to the variance of theoutput 429.

In one embodiment, the following algorithm is used to initialize bothphase shifters 405 and 406, to assure 90-degrees. Again, the algorithmsare not limited to the examples below and a variety of other algorithmscan be used.

For the initialization of both 405 and 406, the following actions can betaken:

1. Adjust control lines 416 and 417 at the same time via controller 454,to cancel the covariance of the two outputs 428 and 429 while, (for theanalog self-homodyne embodiment only) minimizing the expected valueE{((v₁-E{v₁})²-(v₂-E{v₂})²)²}, where v₁ is the measured sampled set atthe output 428, v₂ is the measured sampled set at the output 429.

In the digital implementation embodiment, the algorithm can be performedon any input data, including actual payload (provided that it isscrambled, where all constellation points have equal probability). Inthe analog implementation embodiment, the algorithm can work on trainingsequences, where each symbol is repeated at least for the integrationtime of the slow A/Ds.

The following algorithm can be utilized to track the coupler phase of407, 408, 403 and 404. Again, the present invention is not limited tothese algorithms, which are presented by way of example and withoutlimitation.

To track the coupler phase of 407, 408, 403 and 404, the following canbe utilized.

1. Adjust control line 418 via controller 452, to maximize the varianceof output 428.

2. Adjust control line 419 via controller 452, to maximize the varianceof output 429.

3. Adjust control lines 409 and 415 so that the variance of output 428is equal to the variance of output 429, while

4. Maximizing the sum of the two variances at outputs 428 and 429.

The following algorithm can be utilized to maintain the phase of bothphase shifters 405 and 406. Again, the algorithms are provided by way ofexamples and without limitation. To maintain the phase of both phaseshifters 405 and 406, the following can be utilized.

1. Adjust control lines 416 and 417 at the same time via controller 454,to cancel the covariance of the two outputs 428 and 429, while

2. For the analog self-homodyne embodiment only, minimizingE{((v₁-E{v₁})²-(v₂E{v₂}²)²}, where v₁ is the measured sampled set at theoutput 428, v₂ is the measured sampled set at the output 429.

The Optical Device of the present invention can be utilized forwavelength selectivity by filtering optical signals in the electricaldomain. In various embodiment, the Optical Device of the presentinvention can be utilized in a variety of different of applications andfield, including but not limited to photonics and opto-electronics:communications, LADARs, sensing, and the like. In various embodiments,the Optical Device of the present invention can be utilized to provideinherent frequency selectivity and enables the incorporation ofwavelength agility, by way of illustration into a communications link,without reliance on narrowband tunable optical filters. With the OpticalDevice of the present invention, the preservation of signal phaseinformation, in the electrical domain, enables the implementation of adigital polarization diversity receiver without reliance on opticalcomponents, thus making polarization multiplexing and polarizationagility implementable.

One embodiment of a coherent receiver of the present invention isillustrated in FIG. 4. The tunable laser (not shown) that serves as alocal oscillator impinges the receiver from input 402. It isphase-locked to the incoming signal 401. The PLL can be eitherimplemented optically via the electrical output control signal 429, orin digital signal processor DSP by digital multiplication of the sampledsymbols.

The 90-degrees optical hybrid 410 and the balanced detectors 430, 440act as a mixer, that down-converts the input signal from the opticalband at 401 to the electrical baseband. Because the transformation islinear, all of the filtering, for noise reduction and separation ofcarries, can be performed in the electrical domain, with no reliance onnarrow optical filtering. The electrical output signals from thebalanced detectors 428 and 429 that are connected to the DSP, provide alow-passed sample proportional to the Re{SL*} and Im {SL*} respectively.The samples are processed by the DSP, which further filters out thedesired wavelength, recovers its original polarization, and keeps thetunable laser phase-locked to the incoming signal at 401.

The Optical Device of the present invention can perform wavelengthelectivity by filtering the signals in the electrical domain. A desiredcarrier can be recovered by changing the wavelength of the tunablelaser. Some time, in the order of microseconds, is needed to relock thetunable laser onto the incoming data stream at 401. There is no need fornarrow tunable optical filters who's tuning and settling time are on theorder of several milliseconds, because the local oscillator transfersthe filtering function linearly to the electrical domain.

With the Optical Device of the present invention, no optical componentsare required in order to recover the polarization state of the incomingsignal at 401. The linear transformation of the signal from the opticalto the electrical domain, and its digitization, enables the performanceof digital polarization compensation (in the DSP), thus avoiding theproblematic use of slow, non-endless optical polarization compensators,and the incurring losses.

The Optical Device of the present invention can be utilized incommunication application where spectral efficiency is critical.Analysis, not shown, for signal source from fiber and for free-spacesignal sources shows that the coherent detection described above doesnot require optical channel pre-filtering. The filtering is carried outby the receiver itself. No optical filters are used to separate carriersin a receiver application (since the de-multiplexing is not Lamdabased). Therefore, the communication channels can be packed moreclosely, relying on the disclosed electrical filters to perform theseparation. Calculation results show that even when 160 channels arepresent in the optical filter passband, the penalty in the receiver isminimal. Accordingly, the wavelength selection system discussed abovecan indeed enable the direct electronic filtering of a heavily populatedC-band with hundreds of channels hopping simultaneously, delivering acapacity of 6.4 Tbps.

The Optical Device of the present invention can be utilized forcompensation of communications errors caused by angular misalignment ofthe transmitter and receiver apertures and by the light dispersion inthe transmission medium.

Inter-symbol interference (ISI) is result of spreading of the signalpulse's time slot to its neighboring pulses' time slots. Pulse spreadingcan be caused by dispersion of the transmission medium such as opticalfibers. In free-space optical communications, the dispersion of thetransmission medium is negligible. However, pulse smearing at thereceiver occurs as a result of relative angular misalignment between thetransmitter and receiver aperture due to beam steering, as illustratedin FIG. 7. The relative orientation of the transmitter and receiverboresights cause a “smearing” of the pulse by Δ=D (sin θ−cos tan(θ−θ′))/(c T)=D (sin θ′/cos (θ−θ′)/(c T) (normalized to the pulselength), where D is the aperture length, c is the speed of light, T isthe symbol repetition interval, θis the transmission angle (fromboresight) and θ′ is the receiver to transmitter boresight angledifference as shown in FIG. 7.

With the Optical Device of the present invention a variety of differentapproaches can be utilized to overcome ISI of the optical pulsesincluding but not limited to, (i) implementing pulsed LO at a coherentreceiver to reduce the effect of the ISI compared with CW LO, (ii)digital signal processing, in particular the use an adaptive transversalfilter as a channel equalizer, and the like.

In one embodiment of the present invention, a pulsed local oscillator(LO) samples the input optical signal, and captures and amplifies thesignal power within a narrow time window defined by the pulse width ofthe pulsed LO. As a result, a narrower signal pulse is produced comparedwith cw LO. The signal pulse is therefore effectively reshaped by thepulsed LO. Such pulse reshaping are possible with cw LO only. Thereshaped signal pulses have reduced ISI degradation compared with signalpulses detected using cw LO as it is shown in the results of simulationexperiments presented in FIGS. 8(a) and 8(b). FIG. 8(a) shows the eyediagram as the result of the signal detection with cw local oscillator.The delay is 106.1 ps. The signal was modulated with data usingquadrature phase shift keying (QPSK) format. FIG. 8(b) shows the sameresults but with the pulsed LO used.

The use of pulsed LO as a time-gated amplifier for coherent detectionprovides the following, (i) robustness to inter-symbol interference as aresult of pulse reshaping, with the pulsed LO enabling temporal samplingof the input signal to time gate and reshape the input signal pulse,(ii) performs time filtering of unwanted optical signal from energyspreading from adjacent pulses.

If the input signal is amplified, for example by EDFA with addedbroadband ASE noise, the pulsed LO can perform time-domain filtering andsuppress signal-spontaneous beat noise. This is not achievable with aspectral filter. The pulsed LO acts like an optical gate when the pulsedLO is absent the photocurrent is very small or zero because thephotocurrent is proportional to the square root of the product of the LOpower and the signal power for balanced detection. Noise, such as ASE inthe input signal, is cut off when the pulsed LO is off. ASE is reducedby the duty cycle of the pulsed LO. With pulsed LO of the presentinvention there is, among other things, (i) a reduced saturation effectof the photodector compared to cw LO, (ii) an improvement in sensitivitydue to higher peak power, (iii) an adaptation to coherent detection ofOTDM signal, (iv) an adaptation to optical sampling, optical ADC, andoptical demultiplexing, and the like.

Because the optical signal is down-converted linearly to the electricalbaseband and digitized, digital processing methods used in RF systemscan be utilized for implementation of the Optical Device of the presentinvention in DSP. In one specific embodiment, the Optical Device of thepresent invention is utilized for communication channel equalization andcompensation for linear channel distortion, including but not limited tochromatic dispersion and PMD in fiber, for atmospheric effects in freespace communications, and the like. In one embodiment, use of theOptical Device of the present invention provides an improvement of thesignal BER due to the beam steering equalization when the equalizationDS is applied.

Referring now to FIG. 9, a block diagram of a communication system 500is shown, where 502 and 504 are transceivers that include transmitters506, 508 and coherent receivers 510 and 512, correspondingly.Transceivers 502 and 504 are located at certain distances from eachother, and a light beam 514 passes a transmission medium 516 includingbut not limited to, fiber, air, space, and the like before it reachesthe opposite transceiver 502 or 504 respectively.

In one embodiment, illustrated in FIG. 10(a), transmitters 506 and 508from FIG. 9 are optical devices as disclosed in application Ser. No.10/613,772, filed Jul. 2, 2003, incorporated by reference, and include,a first Mach-Zehnder modulator 518 that produces a first output, and asecond Mach-Zehnder modulator 520, which produces a second output. Thefirst and second Mach-Zehnder modulators 518 and 520 are coupled to aninput splitter 522. A combiner 524 combines the first and second outputsfrom first and second Mach-Zehnder modulators 518 and 520. A phaseshifter 526 is coupled to the first and/or second Mach-Zehndermodulators 518 and 520. The first Mach-Zehnder modulator 518, secondMach-Zehnder modulator 520, input splitter 522, combiner 524 and thephase shifter 526 are each formed as part of a single chip made ofelectro-optical material.

In another embodiment, illustrated in FIG. 10(b) transmitters 506 and/or508 are quadrature modulators operating with the light in two(orthogonal) polarization states. The quadrature modulator, operatingwith two polarization states of light, is disclosed in the Ser. No.10/613,722 and includes two quadrature modulators 528 and 530 integratedin one chip. Each modulator 528 and 530 operates with the light of oneparticular polarization state. A beam splitter 532 is used to separatethe light with orthogonal polarizations at the entrance of the deviceand direct each polarization light into a separate modulator. A combiner534 is used to combine back after modulation the light with differentpolarizations. The resulting output signal consists of twoquadrature-modulated signals, each of different (orthogonal)polarization.

In one embodiment, communications system 500 is WDM bi-directionaloptical communications system is disclosed with at least 2 bits/s/Hzspectral efficiency using QAM modulation format andpolarization-division-multiplexing. A return-to-zero (RZ) coding of theQAM signal is used to enhance transmission performance and receiversensitivity. By way of illustration, and without limitation, the channelspacing can be 25 GHz. For fiber communications, a fiber bandwidth ofapproximately 200 nm (1450-1650 nm) can be utilized to provide acapacity of 100 Tbits/s or 0.1 Petabits/s using communications system500 with 1000 wavelength channels. Coherent homodyne detection of theQAM channels, using analog and/or digital signal processing forpolarization control as well as phase and frequency synchronization ofthe local laser, provides high receiver sensitivity performance.

In another embodiment, the Optical Device of the present invention isused for coherent detection of range/velocity and orientationmeasurement in optical communications links optical communicationsbetween moving platforms. This provides a determination of transmitterand receiver velocities, position and maneuver, and eliminates the errorassociated with this movement.

The present invention can provide range, velocity, and orientationmeasurement as a byproduct of a fully coherent digitalfree-space-optical communication link 600, illustrated in FIG. 11. Inthis embodiment, there is no need for a separate ‘range finding’apparatus. The coherent transmitter 602 and receiver 604 at each side ofthe optical communication link are part of a digital loop. The frequencyof the two lasers 606 and 608 at the two ends of the coherentcommunication link 600 and are locked to the same atomic line source610, which may be provided by GPS, within a predetermined frequencydeviation that corresponds to the system Doppler shift velocityresolution. A homodyne coherent optical receiver consists of an opticalmixer (90-degrees optical hybrid 612 followed by photodetectors 614 and616) with electrical outputs 618 and 620, linearly down-converting theinput signal from the optical band to the electrical baseband. Theoptical mixer electrical output signals 618 and 620 are connected to aDigital Signal Processor (DSP) 622 which process samples proportional tothe real and imaginary portions of the incoming signal.

The samples are processed with DSP 622, which in addition to recoveringthe incoming message data, performing the following, (i) calculation ofthe incoming signal phase deviation which corresponds to the Dopplershift, (ii) recovers the incoming signal original polarization ascompared with the local laser polarization, (iii) extracts the GPS timestamps embedded in the incoming signal data stream and subtracts themfrom the locally generated atomic clock or GPS time ticks, (iv)optionally keeps tunable laser 608 that serves as a local oscillatorbeing phase-locked to the incoming signal at 624.

Lasers 606 and 608, from FIG. 11, at both ends of the communication linkare locked to the same atomic line within 1 MHz. A 1 MHz Doppler shiftcorresponds to a velocity of 0.5 meter per second, which could be thebase system resolution. The following algorithms, by way of illustrationand without limitation, can be used to calculate the velocity andacceleration.

In one algorithm, DSP 622 detects the frequency difference Δf_(dop) (dueto the Doppler shift) between the incoming signal 622 and the locallaser radiation 626, which are locked to the same atomic line. As anexample, Mach one relative velocity corresponds to Δf_(dop)=600 MHz. Theoptional DSP PLL 628 is activated in order to lock the local laser intoincoming signal frequency shift so that the message data will be decodedproperly.

In a second algorithm, transmitter 602, at one end of communication link600, starts sending a predetermined BPSK sequence of zeros and ones, b₁,b₂, b₃ . . . . DSP 622 knows and recognizes this sequence. DSP 480detects the phase sequence using the 90-degrees optical hybrid 612, andthe balanced detectors 614 and 616, and obtains the sequence of signals,φ₁, φ₂, φ₃ . . . . Taking the previous example where the maximum Dopplershift, corresponding to Mach 1 is about 600 MHz, and a code rate of 10GBs at least 8 first bits have the original code shifted by some phaseΔφ₀, i.e φ_(i)=Δφ₀+b_(i). This is enough to recognize the beginning ofthe sequence and establish the value of Δφ₀. The original code b₁, b₂,b₃ . . . is recovered. However, as time elapses there is a bit errorwhen the accumulated phase shift between the incoming signal and locallaser reaches 180 degrees (BPSK), resulting in the sequence:

-   -   b₁, b₂, b₃ . . . b_(k−1) b _(k+1). . . b _(2k−1), b_(2k),        b_(2k+1) . . . . The data bit error appears in k-th bit and        disappears in the 2k-th bit only to reappear again in the 3k-th        bit. The following can thus be written: kTb_(i)Δf_(dop)=1/2; and        therefore, The Doppler shift is Δf_(dop)=1/2 kTb_(i). Having        established the Doppler shift, the signal can then be received        for a while in the BPSK format while automatically performing        the correction for it, with the use of efficient phase error        correction schemes such as TCM. By doing this forward error        correction DSP 622 calculates the Doppler shift and determines        exactly where the bit k is. DSP 622 calculates the acceleration        by monitoring the change in the Doppler shift versus time.

In another embodiment, illustrated in FIG. 12, the Optical Device of thepresent invention is used for maneuverability control, generally denotedas 700. In this embodiment, the two lasers 702 and 704 at both ends ofcommunication link 708 are locked to the same atomic line 710 andtransmit polarized light. The receiver 712 employs a polarizationdiversity, phase diversity scheme. In this scheme the incoming signal issplit by polarization beam splitter 712 into two beams with orthogonalpolarization. Signals at two orthogonal polarizations are linearlytransferred to the electrical baseband by two 90-degrees optical hybrids714 and 716. The local oscillator, is in phase, and locked to theincoming signal, as described above in the velocity and accelerationcalculation. The actual implementation of the PLL 718 is done in thedigital part of receiver 720, and eliminates the need for a complexoptical PLL. Because the transformation is linear, all of the filtering,for noise reduction and separation of carriers, can be performed in theelectrical domain, with no reliance on narrow optical filters. Receiver712, based on complex envelope data from two orthogonal polarizations,can track the rates of changes of the polarization and optimizecorrection matrix to recover the original polarization of thetransmitted signal. After polarization is restored, by numericalmethods, the data on each polarization can then be demodulated.Therefore, the homodyne coherent receiver, in conjunction with DSP 720enables the implementation of a digital polarization diversity receiver712 without reliance on optical components.

In one embodiment, DSP 720 detects and corrects the polarizationdifference Ap, due to the source maneuverability, between the incomingsignal and the local laser. The polarization difference is an indicatorsource maneuverability since it is proportional to it.

In another embodiment, lasers 702 and 704, in FIG. 12, are at the bothends of communication link 700 are locked to the same atomic line source710, may be provided by GPS, within a predetermined frequency deviation.Timing signals generated by atomic source 710 or GPS are embedded in apredetermined format in the data stream flowing between the source andthe coherent homodyne receiver 712. DSP 720 recovers the incoming dataand compares the decoded timing signals to the internally generatedtiming signals. Because the incoming signal processing time delay, theoutgoing processing time delay and the speed of light in the link mediumare known, DSP 720 calculates the time difference between the source andthe receiver time ticks, and establishes a range finding system.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

1. An optical device for receiving and demodulating of a pulsed incomingoptical signal transmitted through an optical link, comprising: a90-degrees optical hybrid having a first and a second inputs, a first, asecond, a third and a fourth outputs, a pulsed local oscillator with aduration of each optical pulse equal to T₁, the pulsed local oscillatorconnected to the 90-degrees optical hybrid via the second input, fourbalanced photodetectors, connected to the first, the second, the thirdand the fourth outputs of the 90-degrees optical hybrid, the balancedphotodetectors having electrical outputs, a digital signal processing(DSP) unit connected to the electrical outputs of the balancedphotodetectors, the DSP unit recovering data from the incoming opticalsignal, wherein the 90-degrees optical hybrid is adapted to mix a pulseof the local oscillator with the duration T₁ with an incoming opticalpulse with a duration T₂, entering the 90-degrees optical hybrid via thefirst input, where T₂ is greater than T₁, the 90-degrees optical hybridperforming time-domain filtering of the incoming optical signal.
 2. Theoptical device of claim 1, wherein the optical device is a free-spaceoptical link device.
 3. The optical device of claim 1, wherein theoptical device is a fiber communications optical link device.
 4. Theoptical device of claim 1, wherein the optical device is a satellitecommunications optical link device.
 5. The optical device of claim 1,wherein the optical device is used for optical sampling of the incomingsignal.
 6. The optical device of claim 1, wherein the 90-degrees opticalhybrid is formed as part of a single planar chip made of electro-opticalmaterial.
 7. The optical device of claim 1, wherein the duration of thepulse T₁ is selected small enough to mitigate inter symbol interferencein the incoming optical signal.
 8. The optical device of claim 1,wherein the duration of the pulse T₁ is selected small enough tosuppress signal-spontaneous beat noise of the incoming optical signal.9. A method of receiving an incoming pulsed optical signal, comprising:mixing a pulse with a duration T₂ from the incoming optical signal witha pulse having a duration T₁ from a pulsed local oscillator signal,where T₂ is greater than T₁, time-domain filtering of the incomingpulsed optical signal, recovering data encoded in the incoming signal ina digital signal processing unit.
 10. The method of claim 9, wherein thetime-domain filtering is performed in 90-degrees optical hybrid.
 11. Themethod of claim 9, wherein the data encoded in the incoming signal isquadrature phase shift keying modulated data.
 12. An opticalcommunication system for optical signal transmission, comprising: atransmitter of a quadrature modulated optical signal; and a receiver,the receiver having a 90-degrees optical hybrid with a first and asecond inputs, a first, a second, a third and a fourth outputs, the90-degrees optical hybrid receiving the quadrature modulated opticalsignal via the first input, a pulsed local oscillator with a duration ofeach pulse equal to T₁, the pulsed local oscillator connected to the90-degrees optical hybrid via the second input, four balancedphotodetectors, connected to the first, the second, the third and thefourth outputs of the 90-degrees optical hybrid, the balancedphotodetectors having electrical outputs, a digital signal processing(DSP) unit connected to the electrical outputs of the balancedphotodetectors, the DSP unit recovering data from the incoming opticalsignal, wherein the 90-degrees optical hybrid is adapted to mix a pulseof the local oscillator with the duration T₁ with an incoming opticalpulse having a pulse duration T₂, where T₂ is greater than T₁, the90-degrees optical hybrid performing temporal filtering of the incomingoptical signal to improve system performance.
 13. The system of claim12, wherein the system performance is improved by reduced saturationeffect of the photodetectors.
 14. The system of claim 12, wherein thesystem performance is improved by reducing an amplified spontaneousemission noise in incoming optical signal.
 15. The system of claim 12,wherein the system performance is improved by reduction of the incomingsignal degradation caused by inter-symbol interference.
 16. The systemof claim 12, wherein the system performance is improved by compensatingof communications errors caused by angular misalignment of thetransmitter and receiver in free space optical communications.
 17. Thesystem of claim 12, wherein the system performance is improved byreducing inter symbol interference caused by fiber dispersion in fibercommunications.
 18. The system of claim 12, wherein the transmittercomprises: a first Mach-Zehnder modulator that produces a first output;a second Mach-Zehnder modulator that produces a second output; asplitter coupled to the first and second Mach-Zehnder modulators; acombiner that combines the first and second outputs; and a phase shiftercoupled to the first and second Mach-Zehnder modulators, wherein thefirst Mach-Zehnder modulator, the second Mach-Zehnder modulator, thesplitter, the combiner and the phase shifter are formed as part of asingle planar chip made of electro-optical material.
 19. The system ofclaim 12, wherein the quadrature modulated optical signal has awavelength of between about 1500 nanometers and about 1625 nanometers.20. The system of claim 12, wherein the quadrature modulated opticalsignal has a linear polarization.